The growing diffusion of the high technologies in every field of human activity has highly increased the need of a wide availability of high accuracy time references. Fortunately, the time is the physical measure that can be measured with the highest degree of accuracy, thanks to the atomic clocks of time metrology laboratories, called primary time standards. If the degree of accuracy is not a problem, on the contrary a wide availability of primary standards is a problem. Indeed, primary time standards are generally expensive laboratory instruments kept in very controlled environments to minimize external perturbations. Only in this way, it is possible to achieve the highest degree of accuracy (1 x 10-14, 1 x 10-15). This problem is solved by time dissemination, a set of tools and operations that allow to keep synchronized to the primary time standards a virtually unlimited number of clocks of any type, spread everywhere.
The importance of having accurate time references is entertained by the law of the states that charge scientific organizations, generally of government origin, with the implementation and keeping of primary standards and with the dissemination of the legal time. This time is the UTC time plus the time zone difference from the Greenwich meridian. The legal time of Italy is 1 hour in advance of UTC time (UTC +01:00).
In many countries, to make a better use of day-light, every year in the period spring-fall the day-light saving time in advance of 1 hour from legal time is adopted (in Italy UTC +01:00), this time is also called summer legal time. Often, one can ear of legal hour to indicate the day-light saving time and of solar hour to indicate the legal time, even if both are legal times, because adopted by law. The table shows the begin and the end time of the day-light saving time in Italy since year 1998 until year 2001.
|1998||29 March ore 02:00||25 October ore 03:00|
|1999||28 March ore 02:00||31 October ore 03:00|
|2000||26 March ore 02:00||29 October ore 03:00|
|2001||25 March ore 02:00||28 October ore 03:00|
At the beginning of the day-light saving time the clocks are advanced of 1 hour, at the end clocks are delayed of 1 hour. Also the hour for time change is established to keep the coherence of time scales for those activities especially sensitive like the transports. For example, let think to the railroad traffic based on accurate timetables.
The past and the future of the day-light saving time in Italy can be see in the day-light saving time in Italy from 1916 to 2006.